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A Nose for Sexual Preference

A new study indicates that homosexual men and heterosexual women respond to the same male scent molecules. Although small in scale, the research could indicate one way in which sexual preference may be encoded in the brain.

When it comes to choosing mates, most animals rely on sex-specific scent molecules called pheromones that activate their brain's mating center. The equivalent region in humans--the front end of the hypothalamus--is also involved with sexual arousal but is not directly wired to the nose. So the role of pheromones in people is less clear. The best candidates for human pheromones are a modified form of testosterone, called AND, that wafts off men through their sweat, and a molecule related to estrogen called EST that ends up in women's urine. Some studies have shown that male sweat activates the hypothalamus and can trigger sexual arousal and ovulation in women, but others have yielded inconclusive results.

To get a better handle on whether AND and EST are cupid molecules, a team of researchers led by Ivanka Savic Berglund, a neuroscientist at the Centre of Gender-Related Medicine at the Karolinska Institute in Stockholm, Sweden, gave 36 people tiny whiffs of the molecules while scanning their brains with positron emission tomography (PET), which detects increased blood flow. The researchers then compared the PET responses of 12 heterosexual women, 12 heterosexual men, and 12 homosexual men.

The team's results suggest that, when it comes to deep-seated sexual attraction, the nose knows. A whiff of AND increased blood flow to the primitive mating center of homosexual men and heterosexual women, but did nothing for heterosexual men, the team reports online this week in Proceedings of the National Academy of Sciences. In contrast, EST activated the same part of the brain in heterosexual men, but left the brains of homosexual men and heterosexual women cold.

The study is important because it identifies a way that sexual preference might be encoded in the brain, says Dana Small, a neuroscientist at Yale University Medical School in New Haven, Connecticut. But she cautions that rather than being born with a nose for the male pheromone, homosexual men may have become sensitive to AND as a result of sexual relations with men.